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The present invention relates generally to coating particles with thin-film layers, and more particularly with coating particles used to produce optically variable xe2x80x9ccolor shiftingxe2x80x9d pigments, i.e. Fabry-Perot or dielectric optical stacks.
Optically variable pigments (xe2x80x9cOVP""sxe2x80x9d(trademark)) are used in a wide variety of applications. They can be used in paint or ink, or mixed with plastic. Such paint or ink is used for decorative purposes, as well as an anti-counterfeiting measure on currency. OVP""s typically include a number of thin-film layers on a substrate that form an optical interference structure. Generally, a dielectric (spacer) layer is often formed on a reflector, and then a layer of optically absorbing material is formed on the spacer layer. Additional layers may be added for additional effects, such as adding additional spacer-absorber layer pairs. Alternatively optical stacks composed of (high-low-high)n or (low-high-low)n dielectric materials, or combinations of both, may be prepared.
Many different techniques are used to produce the OVP structure. One approach commonly referred to as xe2x80x9croll-to-rollxe2x80x9d coating deposits the layers of materials on a moving plastic sheet (such as polyethylene terephthalate or xe2x80x9cPETxe2x80x9d) that spools off of one roll and onto another. The total coated area is very large, and the thin-film stack has to be separated from the plastic roll to form the pigment. This is often done using an organic solvent, such as acetone, to dissolve a xe2x80x9chard coatingxe2x80x9d layer between the PET film and the thin-film stack. The separated pigment material is then ground and sorted for size for mixing into paints, inks, plastics, cosmetics, coatings, or other media.
However, acetone is undesirable because it is a volatile, flammable, organic compound that can contribute to air pollution and must be handled carefully to avoid affecting the work environment. The cost of the acetone adds to the manufacturing costs, and creates a waste stream for disposal of the acetone. It would also be desirable to avoid the cost of the roll of plastic sheet, which is typically disposed of and recycled, as well as the steps of removing, grinding, and sizing the pigment material.
Another approach is to deposit the color-shifting structure on small metal particles or flakes. In one process, the spacer and absorbing layers are deposited on aluminum flakes, which serve as the reflector in the pigment. The major surfaces of such metal flakes are generally not specular, which can cause dispersion of the light. Metal flakes can also be relatively heavy (dense).
Several techniques have been developed for depositing thin-film layers of various materials. Many deposition apparatus and techniques have been developed for manufacturing semiconductor devices, which often use much thinner films than are desired for manufacturing pigment. For example, the dielectric layer between the reflector and the absorber might be several hundred nanometers thick. Thus, other methods have been needed for forming the relatively thick dielectric layer around an aluminum flake. One process deposits a preliminary thin layer of silicon monoxide under vacuum on the aluminum flake to protect it during a sol-gel deposition step, which is a relatively fast wet chemical deposition process. The flakes are then dried and a thin-film layer of optically absorbing material, such as titanium nitride (xe2x80x9cTiNxe2x80x9d), is vacuum deposited on the flake. Techniques for depositing TiN layers are described in co-pending, co-assigned U.S. patent application Ser. No. 09/685,468 entitled TITANIUM-CONTAINING INTERFERENCE PIGMENTS AND FOILS WITH COLOR SHIFTING PROPERTIES by Phillips et al., filed Oct. 10, 2000, the disclosure of which is hereby incorporated by reference for all purposes.
While the resulting flakes have good optical variance properties, the manufacturing sequence is somewhat involved, particularly having to protect the aluminum flake from the wet processing step, and then having to dry the flakes before subsequent vacuum deposition steps. In addition, the process uses a roll of PET as the substrate, which is subsequently discarded. This approach becomes even more involved if several alternating layers of spacer and absorber materials are to be deposited. Therefore, the sol-gel approach encourages one to limit the optical structure to a single spacer layer.
It would be desirable to provide highly reflective flakes and optically variable pigment particles with high chroma, and to provide apparatus and methods for producing optical thin film stacks on particles that achieve high deposition rates with reduced contamination in the overall processing.
One embodiment of the present invention provides an optically variable pigment particle with a substrate particle surrounded by a reflector layer that is surrounded by a spacer layer and an absorber layer. In another embodiment of the invention, a flake substrate with specular surfaces is surrounded by a highly reflective layer. In a particular embodiment, the flake substrate is a glass flake substrate. In further embodiments, the reflective layer is surrounded by additional layers of materials. In yet another embodiment of the present invention, an interference glass flake is surrounded by an absorber layer.
In another embodiment of the present invention, downstream plasma-enhanced chemical deposition systems with vibrating substrate holders are used to coat substrate particles. The vibrating substrate holder can be a vibrating tray that moves the particles past the deposition zone. In a particular embodiment, the particles are circulated past a downstream plasma source or sources. In other embodiments the particles make a single pass. In alternative embodiments particles fall past a downstream plasma source or sources, and may be circulated or make a single pass. In further embodiments, several plasma sources are provided. The various plasma sources can all deposit the same material, can deposit different materials, or some can deposit material(s), while others perform plasma processes such as cleaning, surface activation, or densification of the particle or deposited layer(s).